In linguistics there is a term for when people who have learned a grammatical rule apply it in a logical way that is still incorrect. For instance, when a child learns that to have previously been seated means that you sat, and that when you are seated you sit, they might say that when you were knitting you have knat. It makes sense but only if you step outside of your assumptions about how your own language works.
That concludes the first education tidbit for today.
The next is on inserting underarm gussets into seamless sweaters such as those designed by Elizabeth Zimmermann (Ravelry Link) Some people are familiar with underarm gussets in Fisherman’s ganseys which traditionally have a drop-shoulder construction. The gussets allow the armhole to be shallower than is normal in a drop-shoulder sweater by providing the extra fabric that is normally created by adding depth to the armhole. This provides a greater range of motion by placing a flexible, stretchy expanse of fabric across an area subject to much stress which will in turn increase the garment’s longevity by reducing wear and tear.

The differences between these two underarm types
As you can see, the traditional seamed underarm relies on the rigidity and sturdiness of its seams to resist the stresses placed on the garment when it is being worn. The gusseted underarm circumvents this need entirely by creating a patch of fabric which will take the majority of the stress from reaching, bending, and the ever-present desire to pull the sleeves down a bit. Between the seams and the gusset, guess which one is most likely to wear out first?
You can begin a gusset in many ways, and I recommend reading Knitting In The Old Way to get a start. On this sleeve I began my gusset by purling two stitches centered at the underarm part of my sleeve. My sleeve is being knit in the round with some worsted weight wool.

2 purled stitches in blue
As you can see, the two purled stitches are in blue while the rest are in red. We well finish knitting the round now and come back to these fellows shortly. Upon reaching the 2 purled stitches on the next round, increase 1 stitch on either side of them in any manner you like that results in a purl stitch. On the next round, work these 4 purled stitches as they are and continue apace. When you arrive at these purled stitches again, increase on either side. Continue like this, increasing on either side of these purl stitches every other row.
I will say now, you may work your gusset in whatever stitch pattern you like though I think the best ones are stockinette, reverse stockinette, and seed stitch since these are among the least bulky and easiest to increase into. You may certainly try linen stitch or heel stitch or whatever suits your tastes, but be sure that they are not too bulky and still allow for stretch in the finished fabric.

One half of one gusset, fin!
How do you know how many to increase to? Elizabeth Zimmermann gives us the answer in her very own EPS – We should have at least 8% of our total stitches. i.e. If you are knitting a seamless sweater (And why shouldn’t you, they are lovely and handsome to wear) and have 200 stitches to go around your body, you should have 16 stitches in your underarm gusset. You can have up to 10% for people who like extra room or who have larger upper-arms than many knitwear designers realize. It is true, none of us are perfectly proportionate which is why I encourage you to do the calculations yourself whenever you desire the best fit.
When you have finished all the knitting on your lovely sweater, be it a cardigan or pullover, you may graft the underarm gussets together like any other seamless underarm. It is true that adding the underarm will add extra stitches to the decreasing of any of Elizabeth’s shoulder designs, but I have found that you can sneakily consume these extra stitches by sometimes knitting 3 together or SSSK in order to do a double decrease which falls in line with your shoulder decreases. Spaced across the entirety of the yoke, they do not present a problem or at least it is so slight that a good wash soon sorts them out.